The invention relates to a method of providing a metal component with a thermally black surface. According to the method, a layer of one or more metal alloys are provided on the metal component. The layer has a thickness of one to a few .mu.m. The layer of metal or metal alloy forms at least one metal compound with a material of the component or with each other. The metal compound is obtained by heating the coated metal component in a substantially nonreactive atmosphere.
The invention also relates to a method of providing a deep-drawn cathode shaft having a thermally black surface at least on the inside of the shaft.
It is generally known that the capacity of metals to absorb and radiate thermal energy can be augmented by providing them with a thermally black surface. For example the so-called shadow mask in a color display tube is blackened so as to increase the heat-radiating capacity. It is also known in cathode shafts to thermally blacken the inner surface and/or the outer surface so as to obtain an indirectly heated cathode having a short warm-up time.
German Pat. No. 868,026 describes a method of providing metals with a thermally black surface. In this patent, a thin, for example approximately 10 .mu.m thickness, layer of aluminum or an aluminum alloy is provided on molybdenum. By heating the coated molybdenum in a nonreactive atmosphere, a rough surface layer is formed of a metal compound consisting of aluminum and molybdenum. The disadvantage of such a thermally black layer (consisting, for example, of Al.sub.3 Mo) is that the aluminum evaporates from the compound at higher temperatures. As a result, the thermally black layer becomes less black over time. Moreover, when such black surface coatings are used in electron tubes, display tubes, and camera tubes, the evaporated aluminum forms a metal mirror elsewhere in the tube, which is not desired.